Electric motors are used in particular for driving fans in a cooling system for cooling heat engines of motor vehicles. It is often a requirement that these electric motors should have different operating modes as a function of the load on the heat engine and of weather conditions. These operating modes correspond to different levels of cooling, and comprise a mode in which the motor is stopped, a mode in which it runs at a nominal speed, and at least one intermediate speed mode.
The specification of French published patent application FR 2 658 962A describes a switching device which enables such different modes to be obtained for a fan motor having four brushes, by supplying these brushes directly at the voltage of the source, that is to say without making use of a resistance for reducing this source voltage. In addition, some cooling systems make use of two motorised fan units, both of which then have to be controlled in different modes.
U.S. Pat. specification No. 4 988 930 describes a system which includes two electric motors together with switching means which provide such control for the two motors. In that specification, a first one of the two motors has two independent windings, with one of these windings being arranged to be supplied by itself at the voltage of a source, by virtue of switching means, so as to run the motor at a high speed. The two windings of the motor can be supplied in series at this source voltage to give low speed working of the motor. The second motor has a single winding which is permanently connected in parallel with the first winding of the first motor, so as to be supplied at the source voltage or at a reduced voltage, so that this motor works at either high speed or low speed at the same time as the first motor. In the second operating mode of this type of known system, only one of the two series windings of the first motor is in parallel with the winding of the second motor, and this gives rise to a lack of equilibrium in the power supply to the first motor.